The invention relates to a method and apparatus for measuring gait kinematics such as, for example, acceleration, velocity and position of gait based on foot movement analysis.
The measurement and characterization of gait (i.e. human or animal) is performed by a wide range of methods. At one end of the scale is the measurement and analysis possibilities found in a well equipped bio-mechanical lab. The equipment in these labs typically includes automated 3D optical measurement systems, force plates and physiological output indicators. The output from these transducers are fed into a central computer that enables a wide range of analysis and display possibilities. At the other end of the spectrum is the simplified analysis performed with a ruler, stopwatch and trained clinical observations.
The reasons determining gait kinematic properties (such as acceleration, velocity and position) range from: (i) personal interest, (ii) training and performance considerations of the serious athlete, (iii) rehabilitation of the disabled or (iv) for the design and analysis of footwear.
From an athletic point of view, runners, joggers and walkers often like to know how far they have journeyed and how fast they have traveled, but have had only limited cumbersome ways to measure distance and speed. Distance can be measured after the fact with a calibrated bicycle or automobile or by traveling on a known premeasured route. For determining one""s speed, a simple approach is to travel a known, fixed distance on a track or road and then record the length of time required to cover the distance. This method suffers from several limitations including (i) limited walking/running routes, (ii) speed indication at measured intervals only and (iii) only an average velocity is determined over the given distance.
There are a number of portable pedometers that attempt to tackle the problem of measuring both distance and velocity. However, they have failed to gain wide spread use, because these devices are essentially limited to stride counting. Distance and speed can only be estimated if stride length consistency is assumed. This approach is inaccurate because an individual""s stride length changes considerably from day to day or even within one session due to changes in terrain, fatigue, interval training, or other factors.
U.S. Pat. No. 3,355,942 discloses a pedometer that counts strides based on compression cycles in a bellows under the heel and then estimates distance based on average stride length. The invention described in U.S. Pat. No. 4,741,001 uses a spirit-biased pendulum to count strides. The pedometer disclosed in U.S. Pat. No. 4,649,552 uses a step sensor sealed into an insole to count strides. The pedometer of U.S. Pat. No 4,651,446 counts strides by detecting flexion of the instep. Other counting pedometers include those under U.S. Pat. Nos. 5,117,444, 5,065,414, 4,855,942, 4,510,704, 4,460,823, 4,371,945, 4,322,609, 4,053,755, 3,818,194 and 3,635,399.
The majority of the patented pedometers are simply different methods of stride counting and do not address the problem of varying stride length. However, a pedometer listed under U.S. Pat. No. 4,371,945 uses ultrasonic emitters and sensors on alternate legs to measure the maximum distance between legs during each stride. While this is a significant improvement, this is only suitable for simple, low-speed gait patterns (no flight stage) and requires two sets of transducers; one on each leg.
U.S. Pat. No. 5,097,706 describes a device for taking measurements of various components of the movement of a horse. The device carries six accelerometers disposed to measure accelerations along the x, y and z axis.
Another U.S. Pat. No. 5,724,265 teaches a device that measures distance traveled, speed and height jumped of a person while running or walking. The device includes accelerometers and rotational sensors.
The broad concept of using accelerometers for determining the velocity and distance traveled, for example by athletes, is also described in German Patent 4,222,373. This patent describes the use of an accelerometer and integration to determine velocity and route or position. This device apparently processes acceleration data continuously and thus has an accumulated error from drift so that in very short period of time, the resulting data contains significant inaccuracies. The inventor indicates that this device is useful for skiers, surfers, sailors, cyclists, etc. and thus is not related to a striding device or for measuring the kinematics of striding and would not be effective for that purpose.
The Russian Patents 862074 and 885879 both by Volkov describe the attempts to overcome accumulated error in acceleration measuring devices by using a bar generator in combination with a summator and integrator. This described device does not make use of updated reference points and is thus also prone to accumulated drift.
A paper entitled xe2x80x9cEstimation of Speed and Inclination of Walking Using Neural Networksxe2x80x9d by Aminian et al., Published in the IEEE, Tiansactions on Instrumentations and Measurements; Volume 44#3, Jun. 1995, describes a portable data logger designed to record body accelerations during walking and uses three orthogonal accelerometers placed on the waistbelt to measure forward, vertical and heel acceleration. By means of neural networks, it correlates the recorded signals to the desired gait velocity and angle of incline. The generality of this method is questionable and no other gait information is produced.
The purpose of the device described herein is to provide a means to measure and display several gait parameters (that may include instantaneous and average accelerations and velocities as well as total distance traveled) by means of a simple, low-cost, portable device that can accommodate a wide variety of gaits and varying stride length. The device can be used for human or animal study.
The present invention measures various results about each individual stride rather than assuming a given fixed length. With suitable signal processing, the device can accurately determine velocity and distance traveled. The present invention can be modified to give many other useful indicators to the user such as pronation angles and impact forces. Because it is based on acceleration measurements and analysis, it inherently contains data that correlate directly to impact forces. When integrated, the acceleration data yields both instantaneous and average velocity. A second integration of these signals yields distance information such as, for example, total distance traveled, stride length and height of foot off the ground. Other relevant pieces of information include stride rate (ie. cadence) and peak foot velocity. The invention also has the potential to measure biomechanic parameters such as force of impact and gait sway and can be used for off-angle feet.
In broad terms, the present invention relates to a method of determining gait kinematics for a subject in each of a plurality of strides comprised during each stride defining a fresh datum plane, determining angles between a pair of accelerometers and said datum plane, said pair of accelerometers being adapted to measure acceleration in two directions, the two directions being separated by a known angle of greater than 0xc2x0, and being adapted to measure acceleration in a plane of motion substantially perpendicular to said datum plane, measuring acceleration in said plane of motion in said two directions, converting said accelerations to provide determination of a gait kinematic result for each said stride.
The two directions are preferably separated by an angle of between about 45xc2x0 to 135xc2x0 and more preferably are substantially mutually perpendicular to facilitate determination of the gait kinematic result.
The gait kinematic result can be, for example, details of foot motion, acceleration in a selected direction, velocity in a selected direction or distance in a selected direction. The selected direction is preferably either, parallel to the datum plane and in said plane of motion or perpendicular to said datum plane and in said plane of motion.
The gait kinematic result can be integrated to provide further gait kinematic results. As an example, acceleration in a selected direction can be integrated to determine velocity in a selected direction. In addition, velocity in a selected direction can be integrated to determine distance traveled in a selected direction.
The fresh datum plane is preferably defined when the pair of accelerometers are at a selected position relative to the datum plane. In particular, preferably, it can be determined that the pair of accelerometers are in the selected position by monitoring for foot impact with a surface just prior to the stance phase of the gait. The impact is defined by, for example, a rapid deceleration as determined by the pair of accelerometers or by a switch etc. actuated by impact. In one embodiment, the fresh datum plane is defined at impact plus 0.1 seconds, which is an estimate of the time, in a normal running stride, when the a sole plane of the foot is at rest on a surface in the stance phase of the gait. At this point, the angle between the accelerometers and the datum plane is reset to its original selected value. The original selected value defines the angle between one of the accelerometers and the datum plane, when the foot is at rest or in the stance phase. For example, where one of the accelerometers is positioned parallel to the datum plane during stance phase, the original selected value will be zero.
The duration of a stride (ie. when a stride begins and ends) can be determined in any suitable way. In one embodiment, the stride is determined to be the activity between when the pair of accelerometers are at a selected position relative to the datum plane. In a preferred embodiment, the beginning and end of a stride are determined by observation of impact between the foot and a surface.
Preferably, the method includes further steps for correcting for drift error.
In one embodiment, the subject""s mass is determined and used to determine impact force.
In another embodiment, the method includes measuring acceleration in a lateral direction out of the plane of motion and converting said accelerations measured by the pair of accelerometers and the lateral accelerometer to provide determination of a gait kinematic result.
Broadly the present invention also relates to a device for measuring gait kinematics comprising means for mounting a pair of accelerometers in a fixed relationship to a datum plane defining surface and said pair of accelerometers being adapted to measure acceleration in two directions, the two directions being separated by a known angle of greater than 0xc2x0, means defining a datum plane for each stride for which said gait kinematics is measured as a plane occupied by said datum plane defining surface when said datum plane defining surface is in a substantially stationary position in a stance phase of said stride, means for determining angular orientation of said accelerometers to said datum plane, means for determining a gait kinematic result based on measurements of acceleration by said pair of accelerometers and said determined angular orientation of said accelerometers to said datum plane.
The two directions are preferably separated by an angle of between about 45xc2x0 to 135xc2x0 and more preferably are substantially mutually perpendicular to facilitate determination of the gait kinematic result. The gait kinematic result can be, for example, details of foot motion, acceleration in a selected direction, velocity in a selected direction or distance in a selected direction. The selected direction is preferably either, parallel to the datum plane and in said plane of motion or perpendicular to said datum plane and in said plane of motion.
The device can preferably include a means for adjusting for drift error correction. A suitable means for drift error correction can include a system for determining the mean signal of any particular gait signal and applying the mean signal to the particular gait signal prior to integration to determine further gait kinematic result. In another embodiment, the means for adjusting for drift error correction is a system for using known physical limits of the derived signal such as, for example, velocity to account for drift.
Preferably said means for determining angular orientation of said accelerometers to said datum plane comprises of a pair of spaced substantially parallel accelerometers mounted in fixed relation to said datum plane defining surface and means for calculating angular orientation based on differences in accelerations measured by said pair of spaced substantially parallel accelerometers.
In one embodiment, useful for gait kinematic studies of off-angle feet, a lateral accelerometer is mounted in a fixed and known relationship to the pair of accelerometers and adapted to measure acceleration in a third direction selected to be different than the two directions and out of the plane of motion. Preferably, the lateral accelerometer is substantially perpendicular to the pair of accelerometers. The device can include a means for converting the acceleration measurements from the pair of accelerometers and the lateral accelerometer with angular orientation information to determine a gait kinematic result.
In broad terms, the present invention also relates to a method of determining gait kinematics comprised during each stride defining a datum plane, determining angles between a pair of accelerometers and said datum plane, said pair of accelerometers being adapted to measure acceleration in two directions, the two directions being separated by a known angle of greater than 0xc2x0 and being adapted to measure acceleration in a plane of motion substantially perpendicular to said datum plane, measuring acceleration in said plane of motion in said two directions, converting said accelerations to provide determination of a gait kinematic result for each said stride and adjusting for drift error correction in said gait kinematic result.
The two directions are preferably separated by an angle of between about 45xc2x0 to 135xc2x0 and more preferably are substantially mutually perpendicular to facilitate determination of the gait kinematic result.
The gait kinematic result can be, for example, details of foot motion, acceleration in a selected direction, velocity in a selected direction or distance in a selected direction. The selected direction is preferably either, parallel to the datum plane and in said plane of motion or perpendicular to said datum plane and in said plane of motion.
The step of adjusting for drift error correction can be carried out in various ways. In one embodiment, the adjusting step is made prior to the step of converting to provide a gait kinematic result while, in another embodiment, the adjusting is conducted after the step of converting. Adjusting can be made by data modification such as in the determination of the accelerations or the gait kinematic result or by modification of the determined gait kinematic result, such as by employing known limitations in the derived signal to adjust for the drift error correction. The adjusting step can provide correction which reduces or removes the drift error.
In one embodiment, the method further comprises one or more integration steps to derive further gait kinematic results from the gait kinematic result. Adjusting for drift error correction can be conducted in any or all of the these integration steps. As an example, in one embodiment, the gait kinematic result is acceleration in a selected direction and the method further comprises integrating said acceleration in said selected direction to determine velocity in said selected direction. In such an embodiment, adjusting for drift error correction can be made by determining a mean acceleration in said selected direction and removing the mean acceleration from the acceleration in said selected direction prior to integrating to determine velocity in said selected direction. This adjusting step can be done in each stride. Mean values from one stride can be used for drift error correction in a subsequent stride.
The method can further comprise integrating said velocity in a selected direction to determine distance traveled in a selected direction and, if desired, drift error correction can be made by determining a mean velocity in said selected direction and removing the mean velocity from the velocity in said selected direction prior to integrating to determine distance traveled in a selected direction.
The datum plane is preferably defined when the pair of accelerometers are at a selected position relative to the datum plane. In particular, preferably, it can be determined that the pair of accelerometers are in the selected position by monitoring for foot impact with a surface just prior to the stance phase of the gait. The impact is defined by, for example, a rapid deceleration as determined by the pair of accelerometers or by a switch etc. actuated by impact. In one embodiment, the fresh datum plane is defined at impact plus 0.1 seconds, which is an estimate of the time, in a normal running stride, when the a sole plane of the foot is at rest on a surface in the stance phase of the gait. At this point, the angle between the accelerometers and the datum plane is reset to its original selected value. The original selected value defines the angle between one of the accelerometers and the datum plane, when the foot is at rest or in the stance phase. For example, where one of the accelerometers is positioned parallel to the datum plane during stance phase, the original selected value will be zero. The datum plane resetting can alternatively use gait speed or foot plant duration information to modify the fresh datum plane selection.
In another embodiment, the method further comprises converting said accelerations to provide acceleration substantially parallel to the datum plane and integrating said acceleration substantially parallel to the datum plane to define stride velocity and, if desired, drift error correction can be made by determining a mean horizontal acceleration and removing the mean acceleration substantially parallel to the datum plane from the acceleration substantially parallel to the datum plane prior to integrating to determine stride velocity.
In one embodiment, the method further comprises integrating said velocity in a selected direction to define distance in said selected direction. Drift error correction can be made by determining a mean velocity in a selected direction and removing the mean velocity in a selected direction from the velocity in a selected direction prior to integrating to determine distance in a selected direction.
Preferably said velocity in a selected direction is averaged over a plurality of strides to provide average velocity.
In another embodiment, the step of adjusting for drift error correction employs known limitations in the derived signal. As an example, in a preferred embodiment the gait kinematics for velocity substantially parallel to the datum plane are determined and the velocity is adjusted such that no velocity value is negative, as velocity values are limited to a value greater than or equal to zero.
Preferably said datum plane is defined by the position of a sole plane when said sole plane is at rest on a surface in a stance phase of said gait and wherein said pair of accelerometers are positioned in fixed relationship to said sole plane.
Preferably said step of determining angles of a pair accelerometers is based on measurements of a pair of spaced substantially parallel accelerometers positioned at a selected angle to said sole plane.
In one embodiment, the method determines the gait kinematics in each of a plurality of strides and a fresh datum plane is defined for each stride.
Broadly the present invention also relates to a device for measuring gait kinematics comprising means for mounting pair(s) of accelerometers in a fixed relationship to a datum plane defining surface and said pair of accelerometers being adapted to measure acceleration in two directions, the two directions being separated by a known angle of greater than 0xc2x0, means defining a datum plane measured as a plane occupied by said datum plane defining surface when said datum plane defining surface is in a substantially stationary position in a stance phase of said stride, means for determining angular orientation of said accelerometers to said datum plane, means for determining a gait kinematic result based on measurements of acceleration by said pair of accelerometers and said determined angular orientation of said accelerometers to said datum plane and means for determining for drift error correction.
The two directions are preferably separated by an angle of between about 45xc2x0 to 135xc2x0 and more preferably are substantially mutually perpendicular to facilitate determination of the gait kinematic result.
The gait kinematic result can be, for example, details of foot motion, acceleration in a selected direction, velocity in a selected direction or distance in a selected direction. The selected direction is preferably either, parallel to the datum plane and in said plane of motion or perpendicular to said datum plane and in said plane of motion
The means for adjusting for drift error correction can include a system for determining the mean signal of any particular gait signal and applying the mean signal to a selected signal prior to integration to determine further gait kinematics. In another embodiment, the means for adjusting for drift error correction is a system for using known physical limits of the derived signal such as, for example, velocity to account for drift
Preferably said means for determining angular orientation of said accelerometers to said datum plane comprises of a pair of spaced substantially parallel accelerometers mounted in fixed relation to said datum plane defining surface and means for calculating angular orientation based on differences in accelerations measured by said pair of spaced substantially parallel accelerometers.
Preferably said device further comprises means for converting the accelerations in two directions to obtain acceleration in said selected direction.
Preferably said device further comprises means for converting the acceleration in said selected direction to velocity in said selected direction or to distance in said selected direction by means of integration.
In accordance with another broad aspect for the present invention, there is provided a device for measuring gait kinematics of a stride in a subject having an foot comprising means for mounting a pair of accelerometers in a fixed relationship to a datum plane defining surface and said pair of accelerometers being adapted to measure acceleration in two substantially parallel directions about an axis of rotation defined by movement of the foot, means defining a datum plane measured as a plane occupied by said datum plane defining surface when said datum plane defining surface is in a stationary position in a stance phase of said stride, and means for calculating angular orientation based on differences in accelerations measured by said pair of accelerometers.
Preferably the axis of rotation is substantially parallel to the subject""s sagittal plane and to the datum plane defining surface and, in a particularly preferred embodiment, is that axis about which the foot pronates.
In accordance with another broad aspect, a method of determining pronation characteristics of a foot during a stride is provided comprising during each stride defining a datum plane, determining angular acceleration about an axis about which a foot pronates, converting the angular acceleration relative to the datum plane to determine an angle of pronation for the foot.